TM 55-4920-384-13&P
ZENITH 87A8 CARBURETOR
FOR WISCONSIN MOTOR CORPORATION
orifices are located. All fuel for idling and part throttle
The Zenith 87-Series is a horizontal carburetor with a
operation is supplied through the main jet. Fuel from the
concentric fuel bowl. It is a "balanced" carburetor, because
float chamber flows through the main jet into the metering
all air for fuel chamber and metering well ventilation and
well. Fuel for idling is drawn from this well through the
idling must come through the air cleaner. Air cleaner
calibration, or metering orifice, in the center of the idling
restrictions have a minimum influence on the fuel-air ratio
jet. As the fuel reaches the idling channel it is mixed with
when a carburetor is thus "balanced".
air. which is admitted through a calibrated orifice in the
The main jet and discharge jet are centrally located.
channel from the inside of the air intake to form an
The metering well which completely surrounds the dis-
emulsion. This emulsion is discharged into the air stream,
charge jet is in the center of the fuel bowl assembly. This
to form the idling mixture, through two holes, one of
construction permits extremely high angle operation in
which is controlled by the idle adjusting needle. Turning
any direction.
the adjusting needle counter-clockwise (out) permits more
The venturi, which is part of the throttle body casting,
of the emulsion to reach the air stream and make the
measures the volume of air that passes through the car-
idling mixture richer while turning the needle in (clock.
buretor. In selecting the venturi size, the smallest size that
wise ) cuts off the amount of the emulsion reaching the air
will permit full power development should be used.
stream and makes the mixture leaner.
HIGH SPEED SYSTEM. As the throttle is opened, the
suction on the idling system diminishes, but the increased
volume of air entering the engine through the venturi
creates sufficient vacuum (suction) on the discharge jet to
draw an emulsion of fuel and air from the metering well
which receives its fuel from the main jet and its air from
the well vent. The flow characteristics of the discharge jet
FUEL SUPPLY SYSTEM
FUEL SUPPLY SYSTEM. Fuel under normal pressure
entering the float chamber through the fuel valve seat is
controlled by the twin float which, moving on its axle,
closes the needle valve when the fuel reaches the proper
HIGH SPEED SYSTEM
level in the bowl.
IDLING SYSTEM. At idling speeds the throttle plate is
are influenced by the size, location, and number of holes in
almost closed, thus a very high suction exists at the edge
the sides of that part of the jet which is in the metering
of the throttle plate. At this point the idle discharge
well, as well as by the sizes of the discharge jet orifice,
the size of the main jet, and the size of the well vent. The
well vent is located in the air intake and permits air to
enter the top of the metering well around the outside of
the discharge jet. The flow of fuel through the main jet is
controlled by the main jet adjustment.
CHOKE SYSTEM. Starting a cold engine requires a much
richer mixture of fuel and air. Moving the choke lever to
close the choke plate restricts the air entering the car-
buretor, except at the pitot tube to the bowl vent, and
increases the suction on the idling system which makes the
mixture richer.
STARTING THE ENGINE. Before cranking the engine,
the carburetor throttle should be opened a little to expose
both idle discharge ports to suction. The choke should be
fully closed until the engine starts, then opened a little to
prevent stalling from being over-choked, then when the
engine is fully warmed up the choke can be returned to
wide open position and the throttle closed to the idling
position.
ADJUSTMENTS. Adjust the throttle stop screw to obtain
the desired idling speed by turning the screw in (clock-
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